Statistics, definitions and calculations Flashcards
How do you work out sensitivity?
Number of true positives/ all those with disease.
How do you work out specificity?
Number of true negatives/ all those without disease.
How do you calculate the negative predictive value?
Number of true negatives/ all those that test negative.
How do you calculate positive predictive value?
Number of true positives/ all those that test positive.
How do you calculate the likelihood ratio for a positive result?
The chance that a test is positive if a patient has the disease/ the chance that the test is positive if the patient is well.
How do you calculate the likelihood ratio for a negative result?
The chance that a test is negative if a patient has the disease/ the chance that the test is negative if the patient is well.
The larger the positive likelihood ratio….
… the greater the chance that you have the disease is your test is positive.
The smaller the negative likelihood ratio…
… the lesser the chance that you have the disease if your result is negative.
How do you calculate the chances of having a disease after a test?
The chances of having the disease before the test x likelihood ratio.
What is a nomogram?
A way of relating the likelihood ratios to the pre and post test probabilities.
What does the vertical line on a forrest plot represent?
The line of null effect.
What does the horizontal axis on a forrest plot represent?
The statistic that the studies are profiled to show.
Where is the line of null effect placed on a forrest plot?
At the value where there is no association between an exposure and outcome or no difference between 2 interventions.
In which cases will the line of null effect be placed at 1?
For relative statistics such as an odds ratio or a relative risk as these have a null effect value of 1.
In which cases will the line of null effect be placed at 0?
For absolute statistics such as absolute risk, ARR or SMD (standardised mean difference) as the null difference value for these is 0.
What does each horizontal line put onto a forrest plot represent?
A separate study which is being analysed.
Each study result being represented on a forrest plot has 2 components to it, what are they?
1) A black square box.
2) A horizontal line.
What does each individual black square box represent on a forrest plot?
A point estimate of the study result and the size of the study.
The bigger the box, the more participants in the study.
What does each individual horizontal line on a forrest plot represent?
The 95% confidence intervals of the study.
Each end of the line represents the boundaries of the confidence intervals.
What does the term ‘95% confidence interval’ mean?
The range of values within which you can be 95% certain the true value lies.
What does it mean if the horizontal line of a study crosses the line of null effect?
This means that the null value lies within the confidence interval and hence could be the true value.
**Basically, any study which crosses the line of null effect does not illustrate a statistically significant result.
What is a basic rule of thumb linking the size of a study and the horizontal line of the study?
Often, the bigger the study, the smaller the horizontal line. This means that it is less likely that those studies will cross the line of null effect because the 95% confidence intervals should have a much smaller range.
What is potentially the most important factor to look at on a forest plot?
The diamond at the bottom of the results.
What does the black diamond on a forest plot represent?
The point estimate and confidence intervals when you combine and average all of the individual studies together.